A conventional braking system or a method for operating such a braking system is discussed, for example, in the publication DE 10 2008 004 201 A1, in which a hydraulic layout is described, i.e., a rear axle may be decoupled with a so-called separator valve, whereas publication DE 10 2010 042 995 A1 discusses a method for blending. The teachings of these documents are herewith incorporated by reference in the present application.
A braking system in principle is assumed, as it is schematically depicted in FIG. 1 and as it corresponds essentially to the braking systems described in the aforementioned publications.
The essential components of the braking system shall be only briefly discussed with reference to FIG. 1.
Reference numeral 10 refers to a brake input element, which corresponds to a brake pedal of a motor vehicle. Attached to brake input element 10 is an actuating travel sensor 12, which senses a driver's braking input, when the driver actuates brake input element 10 for braking, i.e., for decelerating the vehicle.
Reference numeral 13 designates a brake booster, which, for example, may be configured, among other things, as a vacuum brake booster, an electrohydraulic or electromechanical brake booster, as this is also known from the related art, brake booster 13 being coupled to a brake master cylinder 14. Brake master cylinder 14 is coupled to a brake medium reservoir 16 for supplying brake medium or brake fluid, which is fillable via a filler neck 18.
Brake master cylinder 14 includes two volume chambers (not visible here), each of the volume chambers being fluidically connected to a feed line or fluid line 28 and 30. Feed lines 28, 30 are feed lines for brake medium or brake fluid, specifically, line 30 for a first brake circuit 24 (on the left side in the illustration of FIG. 1, i.e., when the position of brake master cylinder 14 is assumed to be an imaginary center of the illustration, i.e., thus, the circuit “left” of brake master cylinder 14 is the first brake circuit 24), and line 28 for a second brake circuit 20. In this configuration, first brake circuit 24, when the driver actuates brake input element 10, acts on front wheels 26a, 26b of a front axle (not depicted), each of which are coupled to brake devices 68a, 68b in a known manner. For example, these may be hydraulically actuatable disk-brake units.
In an analogous manner, when the driver actuates brake input element 10, second brake circuit 20 acts on rear wheels 22a, 22b of a rear axle (not depicted) or on (hydraulically actuatable) brake units 38a, 38b coupled thereto, as long as a separator valve 66, which is explained further below, is not closed.
It is understood that additional brake circuits for additional wheel axles are also conceivable.
Situated in the rectangle designated by reference numeral 11 and depicted with a dashed-dotted line is a system 15 of valves (controllable via a control unit not separately depicted here, for example, an ESP unit situated on board of the vehicle), which are fluidically connected to lines 28, 30, and which ensure the hydraulic activation or non-activation of brake units 38a, 38b, 68a, 68b. 
For the sake of conciseness, the conventional operation of valve assembly 15 is not discussed in greater detail here, since this is already described in the aforementioned publications.
In general, the valve system 15 situated in rectangle 11 is referred to as “modulation” (modulation segment), the components situated above as “actuation” (actuation segment), and the components situated below rectangle 11 as “foundation” (foundation segment).
By way of example, however, the components and valves of first brake circuit 24 will be briefly discussed.
Reference numeral 90 refers to a so-called PCR valve (pressure control valve, controllable pressure release valve), which is controllable with respect to the flow rate of fluid, and is closed in the non-activated state.
Reference numeral 66 designates a separator valve, which is open in the non-activated state (“currentlessly open”).
Valves 72a and 72b, which are currentlessly opened, are each inlet valves for the fluid for brake units 68a and 68b on wheels 26a, 26b, whereas valves 86a, 86b are outlet valves of respective brake units 68a, 68b, and are closed in the non-activated state, each of the flow rates of fluid being controllable via a throttle in a known manner. The above applies analogously to second circuit 20.
Also situated in first circuit 24 is a pressure generating device 76 (depicted here are three pump units, whereby there may be at least one or, of course also multiple units), which is designated by reference numeral 44 in second circuit 20. The pump units of pressure generating devices 76 and 44 are driven by a motor 96 having drive shafts indicated extending to the right and left but, for purposes of presentation, graphically interrupted. The pump units of pressure generating devices 76 and 44 may convey brake medium or fluid with the aid of motor 96.
Since the depicted braking system is used in hybrid vehicles or electric vehicles, a (drive motor) electric motor (there may of course also be multiple electric motors) is used when it is used as a generator for recuperation of braking energy and, therefore, electrical energy. This means that with the generator, it is possible also to brake “regeneratively” or “recuperatively”, namely in addition to hydraulic braking, a replacement of hydraulic braking with regenerative braking and vice versa being referred to as “blending.”
For the sake of clarity, the (at least one) electric motor is not depicted in FIG. 1.
In order to achieve a high recuperative efficiency with such a system, it has proven advantageous to introduce a hydraulic “free travel”. Typically, free travel is implemented in brake master cylinder 14, on brake booster 13, or on brake input element 10. This free travel makes it possible to prevent a hydraulic pressure build-up at the so-called coupled axle (in FIG. 1 the axle with wheels 22a, 22b) up to a certain actuating travel. A driver's braking input is, however, nevertheless calculated via the pedal travel, i.e., with the aid of actuating travel sensor 12, and which may then be implemented exclusively and solely via the generator and, therefore, with very high efficiency. In the event the instantaneous generator output alone is unable to achieve the desired deceleration, an additional braking torque is then built up at the brake circuit decoupled via separator valve 66, in order to comply with the driver's deceleration input.
If the recuperative torque of the generator is no longer adequate for the desired deceleration, the missing braking torque is applied hydraulically to the decoupled axle in accordance with the driver's braking input. Very high degrees of efficiency may be achieved with the aid of this system. However, the design implementation of a free travel always entails expenditure and effort. Typically, with respect to the desired free travel, appropriately (model-) adapted “actuation” components must be manufactured. As a general rule, this adapted “actuation” may then no longer be used in conjunction with a standard system.